Procedures for spinning cellulose solutions are known in the art. In this case, a cellulose solution comprising pretreated cellulose, a non-solvent for cellulose, such as water, along with a solvent for cellulose, such as tertiary amine oxide, in particular N-methyl morpholine-N-oxide, in addition to other spinning aids that might be necessary, are prepared to yield a spinnable solution, hereinafter referred to as spinning solution, wherein this mixture only retains its spinnability if kept at a temperature ranging from about 70° C. to 120° C. Spinnability is here the property of spinning the solution into molded parts in a dry-wet extrusion process. EP-A 0 574 870 describes the manufacture of cellulose molded parts out of such solutions, for example.
The above procedures offer clear advantages relative to other procedures for the manufacture of fibers, films and membranes. For example, they enable the manufacture of molded parts superior to conventional molded parts in many respects, such as viscose. The procedure also permits a continuous manufacture of molded parts. In addition, ecological advantages must be highlighted, since essentially no chemicals detrimental to the health or environment are used or precipitated in this procedure for manufacturing solvent-spun cellulose molded parts.
There are also disadvantages to the known procedures and devices for manufacturing solvent-spun cellulose molded parts. The mixtures consisting of cellulose, tertiary amine oxides and water tend to undergo vigorous decomposition reactions at high temperatures, so-called runaways. In addition, decomposition generally sets in after a variable induction period, so that the time and temperature of the runaway reaction are difficult to forecast in practice.
In general, the adiabatic induction period becomes shorter as the temperature of the mixture goes up. The correct selection of temperature and time parameters is pivotal for a safe manufacturing process.
On a production scale, an explosive runaway reaction in the mixture can be expected after about 16 hours for mixtures of cellulose, tertiary amine oxide and water if the mixture is kept at a temperature of about 115° C.
To maintain the spinning solution temperature at a level necessary for spinning in the conveying and handling equipment, e.g., spinning device, mixing container, supply container, lines and other equipment, the individual equipment components are provided with heaters. In this case, subdivision into sectors encompassing individual or several of the above components is a common practice.
Known in the art are various types of heaters that can be attached primarily, but not exclusively, to the outside of the above components. Electric heaters are known, for example. Also known are hot water heaters, in which the necessary warmth is imparted to the spinning solution via hot water streaming through pipes or double walls in the components.
Without wishing to be limited to a single theory, indications are made of an autocatalytic mechanism for the decomposition reaction. Small amounts of Fe(III) ions lead to a noticeable reduction in thermal stability. However, the idea of removing iron ions from the production mixture must be rejected for economic considerations.
The problem involving the spontaneous and explosive decomposition of the mixture requires that special protective measures be taken to prevent serious accidents, and to protect both the equipment for conveying and handling the spinnable cellulose solution, hereinafter called conveying equipment, and personnel, or safeguard them against serious damage or injury.
In prior art, the conveying equipment or its assemblies are provided with conventional and cost-intensive safety devices. These safety devices involve bursting devices that are incorporated at selected locations. Expansion rooms or expansion containers are also used to hold the expanding spinning solution. Such conventional safety devices are not only cost-intensive, but also restricted exclusively in their action to limiting the effects of the spinnable cellulose solution as it passes through so that the conveying equipment is not destroyed. At the same time, the safety of operating personnel is also ensured.
A process for the manufacture of solvent spun cellulose fibres is known from the U.S. Pat. No. 5,401,304, involving transport of the cellulose solution through pipes, whereby an exothermic runaway of the cellulose solution essentially is controlled through regulation of the temperature of the transported solution in dependence of the diameter of the pipe.
The Temperature of the Cellulose solution ist controlled by equipping the pipe with a hollow jacket, containing a circulating heat transfer fluid. At exceeding a limit temperature of the solution to be transported the flow rate is increased or the temperature of the heat transfer fluid is lowered. The temperature of the heat transfer fluid is regulated by a heat exchanger.
Therefore, the disadvantage to the safety devices in prior art is that their action is restricted to limiting the effects of the spinning solution as it passes through, and not limiting the passage of the spinning solution itself.